The principle of secession, of course. After all, he was the author of the American Declaration of Secession from the British empire. In his first inaugural address as president he said:

“If there be any among us who would wish to dissolve this Union or to change its republican form, let them stand undisturbed as monuments of the safety with which error of opinion may be tolerated where reason is left to combat it.”

“Whether we remain in one confederacy, or form into Atlantic and Mississippi confederacies, I believe not very important to the happiness of either part. Those of the western confederacy will be as much our children & descendants as those of the eastern, and I feel myself as much identified with that country, in future time, as with this; and did I now foresee a separation at some future day, yet I should feel the duty & the desire to promote the western interests as zealously as the eastern, doing all the good for both portions of our future family which should fall within my power.”

Addressing the same issue in an August 12, 1803 letter to John C. Breckenridge, Jefferson wrote that if the New England Federalists did secede, creating two American countries, then “God bless them both, & keep them in the union if it be for their good, but separate them, if it be better.”

Jefferson, like all the other founders, understood that the states were all free, independent, and sovereign; that the union was a pragmatic compact between the free and independent states; that the federal government was delegated certain powers by the states for their benefit; and that the citizens of any state had the right to resume those powers should they decide that the union was no longer in their best interest. There was nothing “mystical” or magical or “perpetual” about it. As Jefferson wrote in the last paragraph of the Declaration of Independence:

“That these United Colonies are, and of right ought to be, FREE AND INDEPENDENT STATES; that they are absolved from all allegiance to the British crown and that all political connection between them and the state of Great Britain is, and ought to be, totally dissolved; and that, as free and independent states, they have full power to levy war, conclude peace, contract alliances, establish commerce, and do all other things which independent states may of right do.”​To Thomas Jefferson July 4th was always Happy Secession Day.